// Copyright (c) 2012 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef BASE_TASK_RUNNER_H_
#define BASE_TASK_RUNNER_H_

#include <stddef.h>

#include "base/base_export.h"
#include "base/callback_forward.h"
#include "base/location.h"
#include "base/memory/ref_counted.h"
#include "base/time/time.h"

namespace base {

struct TaskRunnerTraits;

// A TaskRunner is an object that runs posted tasks (in the form of
// Closure objects).  The TaskRunner interface provides a way of
// decoupling task posting from the mechanics of how each task will be
// run.  TaskRunner provides very weak guarantees as to how posted
// tasks are run (or if they're run at all).  In particular, it only
// guarantees:
//
//   - Posting a task will not run it synchronously.  That is, no
//     Post*Task method will call task.Run() directly.
//
//   - Increasing the delay can only delay when the task gets run.
//     That is, increasing the delay may not affect when the task gets
//     run, or it could make it run later than it normally would, but
//     it won't make it run earlier than it normally would.
//
// TaskRunner does not guarantee the order in which posted tasks are
// run, whether tasks overlap, or whether they're run on a particular
// thread.  Also it does not guarantee a memory model for shared data
// between tasks.  (In other words, you should use your own
// synchronization/locking primitives if you need to share data
// between tasks.)
//
// Implementations of TaskRunner should be thread-safe in that all
// methods must be safe to call on any thread.  Ownership semantics
// for TaskRunners are in general not clear, which is why the
// interface itself is RefCountedThreadSafe.
//
// Some theoretical implementations of TaskRunner:
//
//   - A TaskRunner that uses a thread pool to run posted tasks.
//
//   - A TaskRunner that, for each task, spawns a non-joinable thread
//     to run that task and immediately quit.
//
//   - A TaskRunner that stores the list of posted tasks and has a
//     method Run() that runs each runnable task in random order.
class BASE_EXPORT TaskRunner
    : public RefCountedThreadSafe<TaskRunner, TaskRunnerTraits> {
public:
    // Posts the given task to be run.  Returns true if the task may be
    // run at some point in the future, and false if the task definitely
    // will not be run.
    //
    // Equivalent to PostDelayedTask(from_here, task, 0).
    bool PostTask(const tracked_objects::Location& from_here,
        const Closure& task);

    // Like PostTask, but tries to run the posted task only after
    // |delay_ms| has passed.
    //
    // It is valid for an implementation to ignore |delay_ms|; that is,
    // to have PostDelayedTask behave the same as PostTask.
    virtual bool PostDelayedTask(const tracked_objects::Location& from_here,
        const Closure& task,
        base::TimeDelta delay)
        = 0;

    // Returns true if the current thread is a thread on which a task
    // may be run, and false if no task will be run on the current
    // thread.
    //
    // It is valid for an implementation to always return true, or in
    // general to use 'true' as a default value.
    virtual bool RunsTasksOnCurrentThread() const = 0;

    // Posts |task| on the current TaskRunner.  On completion, |reply|
    // is posted to the thread that called PostTaskAndReply().  Both
    // |task| and |reply| are guaranteed to be deleted on the thread
    // from which PostTaskAndReply() is invoked.  This allows objects
    // that must be deleted on the originating thread to be bound into
    // the |task| and |reply| Closures.  In particular, it can be useful
    // to use WeakPtr<> in the |reply| Closure so that the reply
    // operation can be canceled. See the following pseudo-code:
    //
    // class DataBuffer : public RefCountedThreadSafe<DataBuffer> {
    //  public:
    //   // Called to add data into a buffer.
    //   void AddData(void* buf, size_t length);
    //   ...
    // };
    //
    //
    // class DataLoader : public SupportsWeakPtr<DataLoader> {
    //  public:
    //    void GetData() {
    //      scoped_refptr<DataBuffer> buffer = new DataBuffer();
    //      target_thread_.task_runner()->PostTaskAndReply(
    //          FROM_HERE,
    //          base::Bind(&DataBuffer::AddData, buffer),
    //          base::Bind(&DataLoader::OnDataReceived, AsWeakPtr(), buffer));
    //    }
    //
    //  private:
    //    void OnDataReceived(scoped_refptr<DataBuffer> buffer) {
    //      // Do something with buffer.
    //    }
    // };
    //
    //
    // Things to notice:
    //   * Results of |task| are shared with |reply| by binding a shared argument
    //     (a DataBuffer instance).
    //   * The DataLoader object has no special thread safety.
    //   * The DataLoader object can be deleted while |task| is still running,
    //     and the reply will cancel itself safely because it is bound to a
    //     WeakPtr<>.
    bool PostTaskAndReply(const tracked_objects::Location& from_here,
        const Closure& task,
        const Closure& reply);

protected:
    friend struct TaskRunnerTraits;

    // Only the Windows debug build seems to need this: see
    // http://crbug.com/112250.
    friend class RefCountedThreadSafe<TaskRunner, TaskRunnerTraits>;

    TaskRunner();
    virtual ~TaskRunner();

    // Called when this object should be destroyed.  By default simply
    // deletes |this|, but can be overridden to do something else, like
    // delete on a certain thread.
    virtual void OnDestruct() const;
};

struct BASE_EXPORT TaskRunnerTraits {
    static void Destruct(const TaskRunner* task_runner);
};

} // namespace base

#endif // BASE_TASK_RUNNER_H_
